Vocal communications among members of free ranging populations of rats housed in complex laboratory enclosures are either audible to humans, or are above the upper limit of human hearing (ultrasonic). Ultrasonic vocalizations tend to be more prolonged and to form patterned sequences. Rats vocalizing simultaneously are capable of utilizing separate frequency bands so that each signal is distinguishable. Rats required to perform cooperatively appear to exhibit different and, possibly more complex patterns of vocalization, than those rats with free access to food and water resources.